1. Field of the Invention
The present invention relates to a drive apparatus for an optical recording medium such as an optical disk, more specifically relates to a drive apparatus for stabilizing tracking servo by applying a hold voltage to a driver of an optical pick-up upon detection of a defect.
2. Description of the Related Art
In a recording/reproducing apparatus for recording and reproducing information onto/from a disk such as CD (Compact disk) or LD (laser disk), information is accurately read from a recording track on the disk. In order to read the information accurately, a servo technique is used in the recording/reproducing apparatus. More concretely, a closed circuit loop for feedback is formed in the recording/reproducing apparatus, and a speed of a pick-up and disk, a position of a track and a focus of a lens are driven and controlled by the closed circuit loop.
In such a servo technique, an error signal is generated on the basis of a signal input from the pick-up. Further, error signal generating means for generating this error signal has a gain control function for controlling an input voltage so that its value becomes suitable. As this gain control function, an auto gain control circuit is frequently used.
Particularly, a technique for inputting a signal oscillated from a disturbance source as an input signal into a servo circuit and making the auto gain control using the signal is known. This technique is effective because the gain control value can be measured accurately in the servo circuit. When the auto gain control operation is performed before actual reproducing operation, the gain control in the servo circuit can be made accurately, and thus stable servo operation can be realized.
However, when a light beam passes on a black dot, a scratch or the like, a defect occurs in an output signal which is obtained from an optical pick-up by receiving a returned light, and thus a normal RF signal and tracking error signal cannot be obtained. As a result, there is a possibility that the servo occasionally disrupted.
In order to prevent this, a conventional recording/reproducing apparatus is provided with defect detecting means. When a defect is detected by the defect detecting means, a driving signal to be given to the driver of the optical pick-up is held with its level being fixed to DC level just before the detection of the defect. As a result, an influence of the defect can be eliminated, and thus the servo can be stabilized.
However, in the conventional recording/reproducing apparatus, in the case where a 3-beam type optical pick-up is used, the following problem arises. When a beam passes on a black dot or a scratch, a tracking error signal slightly changes into positive or negative signal certainly. Due to the change in the tracking error signal, a pulse-type driving signal is output to the driver of the optical pick-up. As a result, an objective lens of the optical pick-up greatly shifts to a predetermined direction before the holding effect is produced. As a result, the light beam is greatly displaced, and the reproducing performance is deteriorated.
It is an object of the present invention to provide a drive apparatus for an optical recording medium which is capable of stabilizing tracking servo upon detection of a defect even in the case where a three-beam type optical pick-up is used.
A drive apparatus in accordance with the present invention drives an optical pickup for a tracking servo control. The drive apparatus includes an emitting device disposed in the optical pickup for emitting a main beam to a first position on an information track of a recording medium and emitting at least two subsidiary beams to a second position and a third position on the information track of the recording medium, respectively. The second position is different from the first position at least in a tangential direction of the recording medium. The third position is different from the first position at least in a tangential direction of the recording medium. Also, the second position is different from the third position at least in a tangential direction of the recording medium. The drive apparatus further includes: a tracking error signal generating device for generating a tracking error signal using returned light beams of the subsidiary beams from the recording medium; a driving device for moving the first position of the main beam, the second position of one of the subsidiary beams and the third position of the other of the subsidiary beams with respect to the recording medium; a tracking control device for outputting a driving signal based on the tracking error signal to the driving device; and a detecting device for detecting a damaged part on the recording medium. The tracking control device has a pulse signal generating device for generating a pulse signal having a polarity opposite to a disturbance part of the driving signal generated due to at least one of the subsidiary beams passing on the damaged part. The tracking control device then uses the pulse signal as a part of the driving signal.
In the drive apparatus of the present invention, when one of the subsidiary beams passes on a damaged part such a black dot or scratch existing on the recording medium, a defect is formed in reading signals based on the returned light beams of the subsidiary beams from the recording medium. Since the positions of the subsidiary beams are different from each other in at least a tangential direction of the recording medium, the timing that one of the subsidiary beams passes on the black dot (or scratch) and the timing that the other passes on the black dot (or scratch) are different in time. Because the tracking error signal is generated by, for example, subtraction or addition of the two reading signals obtained from the returned light beams of the subsidiary beams, the difference between the timings that the subsidiary beams pass on the black dot (or scratch) generates noise parts in the tracking error signal, and the waveform of each noise part is in the shape of a projection or horn. This means that the noise part has high frequency components. If the frequency components in the noise part exceed a frequency band of the tracking servo, it is difficult to remove the noise part. If the noise part remains in the tracking servo signal, the driving signal is disturbed and a disturbance part is formed in the driving signal. As a result, the driving device moves the positions of the main beam and the subsidiary beams to incorrect positions, and the main beam is thus displaced from a target information track.
To reduce the influence of the disturbance part of the driving signal on the operation of the driving device, the tracking control device has the pulse signal generating device. The pulse signal generating device generates a pulse signal having polarity opposite to the disturbance part of the driving signal, when the damaged part on the recording medium is detected by the detecting device, and then, the tracking control device uses the pulse signal as a part of the driving signal. By supplying the pulse signal to the driving device as a part of the driving signal, the driving device moves the main beam in the direction opposite to the moving direction of the main beam when the disturbance occurs in the driving signal, and thus the main beam is returned to the target information track immediately. Thus, according to the present invention, even in the case where one or both of the subsidiary beams passes on the damaged part on the recording medium, the tracking servo is executed stably.
In the above-mentioned drive apparatus, a signal maintaining device may be added to the tracking control device. The signal maintaining device sets the level of the driving signal at a hold level after the tracking control device uses the pulse signal as a part of the drive signal, and maintains the hold level for a predetermined period of time. The hold level is equal to the level of the driving signal in the state before the detecting device detects the damaged part. Furthermore, the signal maintaining device may maintain the hold level until the detection of the damaged part with the detecting device ends.
While the damaged part is detected by the driving device, the normal tracking error signal cannot be obtained. The signal maintaining device therefore sets the level of the driving signal at the hold level, for example, until the detection of the damaged part ends, after the tracking control device uses the pulse signal as a part of the driving signal. Since the hold level is equal to the level of the driving signal in the state before the detection of the damaged part, the influence of the damaged part can be reduced, so that the stable tracking servo can be achieved.
In the above-mentioned drive apparatus, a pulse setting device for setting a level or a pulse width of the pulse signal may be added. In this case, the pulse setting device may determine the level or the pulse width of the pulse signal on the basis of an amount of a movement of the main beam due to the disturbance part of the driving signal.
When the pulse signal generating device generates the pulse signal, the pulse setting device determines the level or the pulse width of the pulse signal on the basis of the amount of the movement of the main beam due to the disturbance part of the driving signal. Therefore, a pulse signal suitable for reducing the influence of the disturbance part is generated.
In the above-mentioned drive apparatus, a gain detecting device for detecting a gain of the tracking control device and a pulse setting device for setting a level or a pulse width of the pulse signal on the basis of the gain of the tracking control device may be added.
The variation of the gain of the tracking control device causes the increase or decrease of the level of the disturbance of the driving signal, and thus an amount of the main beam movement due to the disturbance of the driving signal increases or decreases. Therefore, the influence of the disturbance of the driving signal can be reduced by determining the level or the pulse width of the pulse signal on the basis of the gain of the tracking control device.
In the above-mentioned drive apparatus, a level detecting device for detecting a level of the tracking error signal and a pulse setting device for setting a level or a pulse width of the pulse signal on the basis of the level of the tracking error signal may be added.
The increase or decrease of the level of the tracking error signal relates to the increase or decrease of the gain of the tracking servo loop, and thus causes the increase or decrease of the disturbance of the driving signal, and thus an amount of the main beam movement due to the disturbance of the driving signal increases or decreases. Therefore, the influence of the disturbance of the driving signal can be reduced by determining the level or the pulse width of the pulse signal on the basis of the level of the tracking error signal.
In the above-mentioned drive apparatus, an RF signal generating device, an RF signal level detecting device and a pulse setting device may be added. The RF signal generating device is a device for generating an RF signal on the basis of a returned light beam of the main beam from the recording medium. The RF signal level detecting device is a device for detecting a level of the RF signal. The pulse setting device is a device for setting a level or a pulse width of the pulse signal on the basis of the level of the RF signal.
The RF signal is generated from the returned light beam of the main beam. The increase or decrease of the level of the RF signal relates to the increase or decrease of the gain of the tracking servo loop, and thus causes increase or decrease of the disturbance of the driving signal, and thus an amount of the main beam movement due to the disturbance of the driving signal increase or decrease. Therefore, the influence of the disturbance of the driving signal can be reduced by determining the level or the pulse width of the pulse signal on the basis of the level of the RF signal.
Moreover, a gain detecting device for detecting a gain of the tracking control device, a level detecting device for detecting a level of the tracking error signal, an RF signal generating device for generating an RF signal on the basis of a returned light beam of the main beam from the recording medium, an RF signal level detecting device for detecting a level of the RF signal, and a pulse setting device for setting a level or a pulse width of the pulse signal on the basis of the gain of the tracking control device, the level of the tracking error signal and the level of the RF signal may be added to the above-mentioned drive apparatus. By determining the level or the pulse width of the pulse signal on the basis of the gain of the tracking control device, the level of the tracking error signal and the level of the RF signal, the pulse signal appropriate for reducing the influence of the disturbance of the driving signal can be generated.
Moreover, in the above-mentioned drive apparatus, the pulse setting device (14) may incudes calculation device for generating a calculation value using a first value representing the gain of the tracking control device, a second value representing the level of the tracking error signal, a third value representing the level of the RF signal, a first coefficient for weighting the gain of the tracking control device, a second coefficient for weighting the level of the tracking error signal, and a third coefficient for weighting the level of the RF signal, and the pulse setting device may set the level or pulse width of the pulse signal on the basis of the calculation value.
The nature, utility, and further feature of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiments of the invention when read in conjunction with the accompanying drawings briefly described below.